With the increasingly widespread use of wind energy installations, these installations have to satisfy more stringent requirements relating to their behavior with respect to the grid system. This is particularly true for those wind energy installation which are intended for connection to a medium-voltage grid system. The grid system connection conditions to be complied with are referred to as “grid codes” of the respective grid system operator. These requirements include, for example, the behavior of the wind energy installations, when a sudden voltage change occurs in the grid system voltage. This requirement, which is also referred to as “voltage ride through”, states that wind energy installations should not be disconnected immediately when a voltage error is present in the grid system, but should remain connected to the grid system at least for a specific time (normally about 150 ms), and should then either feed electrical power into the grid system again as quickly as possible after the grid system voltage returns or should be involved in feeding reactive power for the duration of the grid system error, in order to support the grid system, particularly with respect to the grid system voltage.
In order to verify this required characteristic of the wind energy installations with respect to compliance with the requirements of the grid system operator, test devices are provided. These are based on the knowledge that appropriate measurements are not actually practical on the real public grid system. The test devices are used to simulate appropriate voltage errors in a defined manner, and to allow the behavior of the wind energy installation to be checked. Devices such as these for voltage testing of wind energy installations are known. For example, the disturbance resulting from a grid system undervoltage is simulated by connection of inductors.
EP 1 876 460 A1 discloses a test device which can be connected between a wind energy installation to be tested and the grid system. This has an integrated transformer and a plurality of impedances, which are connected in a matrix form and can be included in the circuit via switches. The device can be used to simulate various faults, such as single-phase or multi-phase shorts between phases or to ground. In this case, the duration and the depth of a voltage dip can be adjusted, but not completely independently of one another.